1. Field of the Invention
This invention relates to an optical disk drive , and more particularly, to an apparatus for adjusting the intensity of a laser beam, or the laser power therein.
2. Description of the Related Art
In an optical disk drive, the optimum laser power during a read operation does not strongly depend on the quality of individual optical disks. The read beam is reflected by the optical disk, or transmitted through the optical disk, but will not cause any change of the composition, shape, optical or magnetic properties of the optical disk.
On the other hand, the optimum laser power during a write operation, since the laser beam projected onto the optical disk will cause the change of the composition, shape, or optical or magnetic properties of the optical disk, significantly depends on the quality or the optical characteristics of individual optical disks. In addition, the optimum laser power during a write operation is influenced by the ambient temperature and by the life of the laser.
For this reason, the read laser power is adjusted in a process for adjusting the intensity of the beam during the manufacturing process of an optical disk drive and placed under feedback control by which a target value, that is, a constant value adjusted during said manufacturing process, is maintained. Also, after an optical disk drive has been manufactured, the write laser power is adjusted each time individual optical disks are loaded in the optical disk drive or regularly after the optical disk drive has been assembled.
The write laser power is adjusted by varying a digital value (DAC value) provided to a digital-to-analog converter (DAC) connected to a voltage-to-current converter (V-1 converter) used as the source which supplies current to the laser. Said DAC value is a digital value for establishing the laser power. An optimum value for the write power is detected following the steps of attempting to write a predetermined test pattern gradually increasing or decreasing a DAC value for each predetermined value, reading said test pattern, finding a DAC value which generates an optimum reading signal, storing the DAC value found in a memory, and finally generating a desired write laser power through the use of the stored DAC value for further writing.
In the conventional method, to perform the above test writing, the initial DAC value and an increment or a decrement (DAC step value) are used for varying DAC values as follows.
First, a reference voltage previously set to a proper value is stored in the optical disk drive. Said reference voltage is compared with the detected voltage of the laser power as the DAC value increases gradually from a very small value, and then the result of comparison is inverted assumed to be said initial DAC value. The DAC step value is a predetermined constant digital value.
In such a conventional apparatus, if the laser efficiency varies from product to product, the value of the laser power generated based on the same DAC step value significantly varies from product to product. On the other hand, an initial DAC value may significantly depend on the variation in the efficiency of the laser as well as the small stability of a reference voltage or a source voltage. Consequently, it becomes hard to obtain an optimum writing power.